This invention relates to N-fluoropyridinium pyridine heptafluorodiborate.
Many fluorinating reagents are known in the art. One type of fluorinating reagent is known as electrophilic fluorinating reagents. This type of fluorinating reagent is characterized by a structure containing an O-F or N-F bond. Electrophilic fluorinating reagents having an N-F bond include 1-fluoro-2-pyridone as taught by S. T. Purrington et al., "1-Fluoro-2-pyridone: A Useful Fluorinating Reagent", J. Org. Chem. 48, 761 (1983); N-fluoro-N-alkylsulfonamides as taught by W. E. Barnette, "N-Fluoro-N-alkylsulfonamides: Useful Reagents for the Fluorination of Carbanions", J. Am. Chem. Soc. 106, 452 (1984); N-fluoroquinuclidinium fluoride as taught by R. E. Banks et al., "Electrophilic Fluorination with N-fluoroquinuclidinium Fluoride", J. of Fluorine Chem. 32, 461 (1986); and N-fluoropyridinium salts as taught by T. Umemoto et al., "N-fluoropyridinium Triflate and its Analogs, The First Stable 1:1 Salts of Pyridine Nucleus and Halogen Atom", Tetrahedron Letters 27(28), 3271 (1986).
N-fluoropyridinium salts have been shown to be stable fluorinating reagents with the ability to fluorinate a variety of organic compounds. For example, T. Umemoto et al., "N-fluoropyridinium Triflate and Its Derivatives: Useful Fluorinating Agents", Tetrahedron Letters 27(37), 4465 (1986) report that these salts are useful in the fluorination of aromatic compounds and the conversion of enol silyl ethers to alpha-fluoroketones. T. Umemoto et al., "Base-Initiated Reactions of N-fluoropyridinium Salts; A Novel Cyclic Carbene Proposed as a Reactive Species", Tetrahedron Letters 28(24), 2705 (1987) report that N-fluoropyridinium salts can also be converted in to useful pyridine derivatives such as 2-chloropyridine.
However, N-fluoropyridinium salts are disadvantageous because the current method for their preparation is hazardous. As taught by the aforementioned first Umemoto et al. article, fluorine is first reacted with pyridine or a substituted pyridine at low temperature to give a pyridine fluoride. In a second step, the pyridine difluoride is converted into an N-fluoropyridinium salt. N-fluoropyridinium tetrafluoroborate and its method of preparation are shown below: ##STR2## See also European Patent Application Publication 204,535. As those skilled in the art known, pyridine difluorides are inherently unstable and have been known to decompose violently even at temperatures below 0.degree. C. If the foregoing preparation method were used on a large scale, the formation of pyridine difluoride, even if temporary, would be extremely hazardous.
In response to the foregoing hazard, we attempted to prepare N-fluoropyridinium tetrafluoroborate without the formation of isolable quantities of pyridine difluoride by reacting fluoride with pyridine-boron trifluoride complex. The resulting product was totally unexpected.